Silicon photonics chips are being developed for high speed interconnects between dies. Waveguides can be built on silicon on insulator (SOI) wafers and can be integrated with CMOS devices. In order to make such integration, though, a connection must be made between an “off-chip” optical fiber and the waveguide structure, itself. Out-of-plane coupling uses an optical grating to couple light from the optical fiber to the waveguide structure, but this limits the optical signal to one wavelength. In-plane coupling, on the other hand, allows broadband transmission (multiple wavelengths, and therefore higher bandwidth). However, achieving high coupling efficiency with in-plane coupling is difficult.
A reason for loss for in-plane coupling is that the core of the optical fiber has a much larger diameter than the Si waveguide structure. The loss can be reduced using an inverse taper on the Si waveguide, for improved coupling. For maximum coupling, it is important to surround the Si waveguide structure with SiO2 or an optical epoxy (same refractive index as SiO2). If the waveguide structure is not surrounded by the appropriate optical material (e.g., SiO2 or optical epoxy), some light will couple into the substrate and the signal will be reduced.
A cavity under the Si waveguide can be filled with optical epoxy to provide the desired optical performance. However, the epoxy is inserted into the cavity after the chip has been joined to an interposer. But, the chip join process includes an underfill material for stability and mechanical strength, which can diffuse into the cavity (instead of optical epoxy) resulting in degraded optical coupling.